The invention relates to nuclear medicine, and more particularly relates to scintillation camera systems for use in nuclear medicine studies. In its most immediate sense, the invention relates to localization of scintillation events which are detected by a scintillation camera system.
In a conventional Anger-type scintillation an array of photodetectors--conventionally, photomultiplier tubes ("PMTs")--is located adjacent a scintillation crystal of e.g. NaI(Tl). While various schemes are used to correct for various nonlinearities in the response of the system, scintillation events are primarily localized by applying a centroid-type calculation to the outputs of the PMTs (which outputs are weighted using a resistor matrix and then added together).
Such localization methodology is most practical when used with analog-type detectors. When digital detector techology is utilized, such techniques are more difficult to implement. It would be advantageous to provide an alternate methodology for localizing scintillation events in scintillation camera detectors of the digital type.
Additionally, centroid calculations and the various linearity correction methods used with them are computationally intensive and require substantial computer resources. It would be advantageous to provide a methodology which did not require intensive calculation and high-performance computer equipment.
The invention proceeds from the known relationship that as distance between a scintillation event and the center of the field of view of the PMT increases, the amplitude of the output signal from the PMT decreases. Thus, there is a known relationship between the amplitude of the output signal and the radial distance between the event and the center of the PMT field of view.
Therefore, when the amplitude of the output signal is known, it follows that the event will be located somewhere on a circular locus of points (hereinafter, "PMT locus"). The center of this PMT locus will coincide with the center of the field of view of the PMT and the radius of this PMT locus will be a function of the amplitude of the output signal. Such a PMT locus can be "looked up" in a lookup table which is programmed into a fast memory (e.g. a ROM), since the location of the center of the field of view of the PMT is known in advance and remains fixed for each PMT and the only variable is the radius of the PMT locus.
Furthermore, when a single event is detected by two adjacent PMTs, the event can be located more precisely. In this case, the event will lie on two PMT loci, each concentric with the center of the field of view of one of the PMTs and having a radius which is a function of the amplitude of the output signal from that PMT. Since the spacing of the PMTs is such that these two PMT loci will intersect at two points, it follows that knowledge of the output signals from two PMTs permits the event location to be localized to either one of these two intersection points.
Let it now be assumed that a single event is detected by three adjacent PMTs. In the general case, the three PMT loci will intersect at six points that can be classified into three pairs of points, each pair of points lying on a common pair of PMT loci. If a line is constructed to connect each such pair of intersection points, the three thus-constructed lines will bound a small area in which the event will be located.
Because the above-described PMT loci are determinable by a method which utilizes a lookup table, and because the geometrical relationship between adjacent PMTs is known in advance and does not change, the location of the event can be determined without using a large number of floating-point operations and is easily implemented in a digital system without requiring substantial computational resources.
In accordance with the invention, the output signals from a plurality of photodetectors (advantageously PMTs) are used to determine the points at which certain two-dimensional curves would intersect. The curves are unambiguously determined by intersections between the photodetector loci. In accordance with the preferred embodiment each such curve is a straight line that is so calculated as to connect a pair of points which lie on the same pair of PMT loci. The thus-determined intersection points bound a relatively small two-dimensional region, in which the event is located.
Advantageously, and in accordance with the preferred embodiment of the invention, this determination is carried out using a mathematical algorithm wherein many terms have been precomputed and have been stored in a fast memory (e.g. ROM) lookup table.
In this way, the invention can easily be implemented in a digital detector, the use of computation is held to a minimum and high-powered computer resources are unnecessary.